The present invention relates to the production of N-methyl pyrrolidone (NMP). In particular, it relates to the use of gamma-butyrolactone (GBL) and mixed methylamines as starting materials, in such a way that the product is obtained by a continuous process in optimal purity and yields.
It is known from the prior art that there exist several processes for the synthesis of N-methyl pyrrolidone, using GBL and one methylamine, monomethylamine (MMA), as starting materials.
In J. Am. Chem. Soc., March 1949, pag. 897, Elvain and Vozza described a synthetic strategy aimed at the production of NMP with GBL and MMA as starting materials that exploited a discontinuous process, and with a double amount of the latter in comparison with his stoichiometric value.
After 4 hours of reaction at 280° C., NMP was recovered by distillation with a 90–93% yield.
In 1936 Spath and Lunder (Berichte 69, pag. 2727) described a similar process wherein a large excess of methylamine (4 mols per mol of GBL) was fed to a discontinuous reactor, with an approximately 90% conversion after 3 hours.
JP-A-72/018751 discloses a discontinuous process for the preparation of NMP by heating GBL with DMA and TMA. The reaction temperature is 230–300° C. The reaction time is in the range of about 0 to 5 hours. After terminating the reaction, the separation of NMP can be performed by subjecting the reaction mixture to fractional distillation.
NMP product purification involved complex dissolution processes, to be carried out in the ether from the effluent of the reaction and the ensuing distillations. In several patents (JD 7 221 420; JP 740025,G; JP 7 420 585; JP 7 642 107) Mitsubishi Chemical Industries Co. Ltd. of Japan described continuous processes for the synthesis of NMP using GBL and MMA as starting materials. These are characterised by reactions with high molar ratios between water and GBL (typically ranging between 3 and 4 mols of water for each mol of GBL) and by the presence of great amounts of MMA (typically with molar ratios ranging between 1.4 and 3 mols of MMA per mol of GBL).
JP-A-01/190 667 teaches the preparation of NMP at 240–265° C. by reacting GBL with MMA, isolating the by-products DMA and TMA from the obtained reaction mixture and recirculating these by-products into the reaction system.
The processes designed by Mitsubishi result to be disadvantageous in terms of the high costs involved with the separation of unreacted MMA and its recovery and with the separation of the water forwarded to the reaction to which synthesis water adds up (one mol of water for each mol of reacted GBL).
To avoid the drawbacks associated with the discontinuous reaction in the presence of excess MMA and water, alternative methodologies have been proposed and these are based on the employment of catalysts. In German Patent No. 2,159,858 owned by Mobil Oil a synthesis with GBL, MMA in the presence of 13X type zeolites is described.
The above processes did not succeed in being applied industrially, as the employment of a catalyst subject to regenerations is disadvantageous in terms of the economic balance of the process as compared with non catalytic processes.
In WO-A-99/52867 the same Applicant discloses a process for the production of N-Methyl Pyrrolidone by reacting in a continuous mode a light excess of pure monomethylamine with gamma-butyrolactone under pressure and without catalyst by using a three stage reactor connected in series.
The present application is an improved low cost process to afford N-Methyl Pyrolidone by reacting gamma-butyrolactone with a mixture of methylamines.
The advantage of this application is the direct link established between a N-Methyl Pyrrolidone plant and a Methylamines plant for a cheaper design. It is known that reacting ammonia and methyl over an amination catalyst produces a mixture of mono-, di- and trimethylamines. The separation of this mixture involves a complex separation sequence which increases significantly the price of pure methyl amines.
So, it is a first advantage of the present application to offer a process in which a low cost mixture of methylamines (coming directly from the ammonia-methanol reaction without a costly fractional distillation step) is used to produce N-Methyl Pyrrolidone.
There is a second advantage of the present application. It is known in the prior art that reacting separately gammabutyrolactone with mono-, di- and trimethylamines produces N-Methyl Pyrrolidone with water and, for the two last amines, water and methanol.
According to the present invention, by using the process disclosed in WO-A-99/52867, the use of a mixture of methylamines allows the production of N-Methyl Pyrrolidone, in absence of added water, still in high purity and yield. The unreacted di and trimethylamines (and methanol) are recycled in the amination section.
The above feature gives way to the production of NMP at low investment costs, using cheaper raw materiali and inferior utility consumptions, compared with the technologies of the previous art.